Heya!

You will probably get different answers from different people. What you should do first, is research focal ratios, and how they affect the image, and the size of the scope. Having the facts ready, will make you feel much better about the decision you make when you start/finish the mirror.
My personal choice would be no faster than an f/5, and probably go even to a slower focal length f/6+ if your scope size isnt an issue.
But, at least persue finding out what is entailed in the different focal lengths first.

Jack McQuaide